Leader Election

Leader Election (LE) problem In a DS, it is often needed to designate a single processor (i.e., a leader) as the coordinator of some forthcoming task (e.g., find a spanning tree using the leader as the root) In a LE computation, each processor must decide between two internal states: either elected (won), or not-elected (lost). Once an elected state is entered, processor is always in an elected state: i.e., irreversible decision In every admissible execution, exactly one processor (the leader) enters an elected state

Leader Election in Ring Networks Initial state (all not-elected) Final state leader

Why Study Rings? Simple starting point, easy to analyze Abstraction of a classic LAN topology Lower bounds and impossibility results for ring topology also apply to arbitrary topologies

Sense-of-direction in Rings In an oriented ring, processors have a consistent notion of left and right For example, if messages are always forwarded on channel 1, they will cycle clockwise around the ring

LE algorithms in rings depend on: Anonymous Ring Non-anonymous Ring Size of the network n is known (non-unif.) Size of the network n is not known (unif.) Synchronous Algorithm Asynchronous Algorithm

LE in Anonymous Rings Every processor runs the same algorithm Every processor does exactly the same execution

Impossibility for Anonymous Rings Theorem: There is no leader election algorithm for anonymous rings, even if the algorithm knows the ring size (non-uniform) in the synchronous model Proof Sketch (for non-unif and sync rings): Every processor begins in same state (not-elected) with same outgoing msgs (since anonymous) Every processor receives same msgs, does same state transition, and sends same msgs in round 1 And so on and so forth for rounds 2, 3, … Eventually some processor is supposed to enter an elected state. But then they all would.

If one node is elected a leader, then every node is elected a leader Initial state (all not-elected) Final state leader If one node is elected a leader, then every node is elected a leader

Impossibility for Anonymous Rings Since the theorem was proven for non-uniform and synchronous rings, the same result holds for weaker models: uniform asynchronous

Rings with Identifiers, i.e., non-anonymous Assume each processor has a unique id. Don't confuse indices and ids: indices are 0 to n - 1; used only for analysis, not available to the processors ids are arbitrary nonnegative integers; are available to the processors through local variable id.

Overview of LE in Rings with Ids There exist algorithms when nodes have unique ids. We will evaluate them according to their message (and time) complexity. Best results follow: asynchronous ring: (n log n) messages synchronous ring: (n) messages, pseudo-polynomial time complexity All bounds are asymptotically tight (though we will not show lower bounds).

Asynchronous Non-anonymous Rings W.l.o.g: the maximum id node is elected leader 8 1 2 5 6 3 4 7

An O(n2) messages asyncronous algorithm: the Chang-Roberts algorithm Every process sends an election message with its id to the left if it has not seen a message with a higher id Forward any message with an id greater than own id to the left If a process receives its own election message it is the leader It is uniform: number of processors does not need to be known to the algorithm

Chang-Roberts algorithm: pseudo-code for Pi

Chang-Roberts algorithm: an execution 1 8 8 Each node sends a message with its id to the left neighbor 1 5 2 2 5 6 3 6 3 4 7 7 4

If: message received id current node id Then: forward message 5 8 1 2 5 8 6 7 3 4 7 6

If: message received id current node id Then: forward message 8 1 7 2 5 6 3 8 4 7

If: message received id current node id Then: forward message 7 8 1 2 5 6 3 4 7 8

If: message received id current node id Then: forward message 8 1 2 5 6 3 4 8 7

If: a node receives its own message Then: it elects itself a leader 8 8 1 2 5 6 3 4 7

If: a node receives its own message Then: it elects itself a leader 8 1 leader 2 5 6 3 4 7

Analysis of Chang-Roberts algorithm Correctness: Elects processor with largest id. msg containing that id passes through every processor Message complexity: Depends how the ids are arranged. largest id travels all around the ring (n msgs) 2nd largest id travels until reaching largest 3rd largest id travels until reaching largest or second largest etc.

Worst case: O(n2) messages Worst way to arrange the ids is in decreasing order: 2nd largest causes n - 1 messages 3rd largest causes n - 2 messages etc. n 1 n-1 2 n-2 n-3

Worst case: O(n2) messages 1 n messages n-1 2 n-2 n-3

Worst case: O(n2) messages 1 n-1 2 n-2 n-3

Worst case: O(n2) messages 1 n-2 messages n-1 2 n-2 n-3

Worst case: O(n2) messages Total messages: n 1 n-1 2 n-2 … n-3

Best case: O(n) messages Total messages: n n-1 1 n-2 2 … 3

Average case analysis CR-algorithm

Average case analysis CR-algorithm Probability that the k-1 neighbors of i are less than i Probability that the k-th neighbor of i is larger than i

Average case analysis CR-algorithm Therefore, the expected number of steps of msg with id i is Ei(n)=P(i,1)*1+P(i,2)*2+…P(i,n)*n. Hence, the expected total number of msgs is:

Can We Use Fewer Messages? The O(n2) algorithm is simple and works in both synchronous and asynchronous model. But can we solve the problem with fewer messages? Idea: Try to have msgs containing larger ids travel smaller distance in the ring

An O(n log n) messages asyncronous algorithm: the Hirschberg-Sinclair algorithm Again, the maximum id node is elected leader 8 1 2 5 6 3 4 7

Hirschberg-Sinclair algorithm (1) Assume ring is bidirectional Carry out elections on increasingly larger sets Algorithm works in (asynchronous) phases Pi is a leader in phase r=0,1,2,… iff it has the largest id of all nodes that are at a distance 2r or less from it; to establish that, it sends probing messages on both sides Probing in phase r requires at most 4·2r messages for each processor trying to become leader

nodes 8 nodes 1 2 5 6 3 4 7

Hirschberg-Sinclair algorithm (2) Only processes that win the election in phase r can proceed to phase r+1 If a processor receives a probe message with its own id, it elects itself as leader It is uniform: number of processors does not need to be known to the algorithm

Phase 0: send(id, current phase, step counter) to 1-neighborhood 1 8 8 1 8 5 2 2 1 2 5 6 5 6 3 4 6 3 3 7 4 7 7 4

If: received id current id Then: send a reply(OK) 8 1 2 5 6 3 4 7

If: a node receives both replies Then: it becomes a temporal leader and proceed to next phase 8 1 2 5 6 3 4 7

Phase 1: send(id,1,1) to left and right adjacent in the 2-neighborhood 8 8 1 8 2 5 5 6 5 6 6 3 7 4 7 7

If: received id current id Then: forward(id,1,2) 8 6 1 8 5 2 5 8 6 7 3 7 6 4 5 7

At second step: since step counter=2, I’m on the boundary of the 2-neighborood If: received id > current id Then: send a reply(id) 8 1 2 5 6 3 4 7

8 1 2 5 6 3 4 7 If: a node receives a reply with another id Then: forward it If: a node receives both replies Then: it becomes a temporal leader 8 1 2 5 6 3 4 7

Phase 2: send id to -neighborhood 8 1 8 2 5 8 7 7 6 3 4 7

At the step: If: received id current id Then: send a reply 8 1 2 5 6 3 4 7

If: a node receives both replies Then: it becomes the leader 8 1 2 5 6 3 4 7

Phase 3: send id to 8-neighborhood  The node with id 8 will receive its own probe message, and then becomes leader! 8 1 leader 2 5 6 3 4 7

In general: n nodes Θ(log n) phases 8 1 leader 2 5 6

Analysis of H&S algorithm Correctness: Similar to C&R algorithm. Message Complexity: Each msg belongs to a particular phase and is initiated by a particular proc. Probe distance in phase i is 2i Number of msgs initiated by a proc. in phase i is at most 4*2i (probes and replies in both directions)

Message complexity Phase 0: 4 Phase 1: 8 … Phase i: Phase log n: Max # messages per leader Max # current leaders Phase 0: 4 Phase 1: 8 … Phase i: Phase log n:

Message complexity Phase 1: 4 Phase 2: 8 … Phase i: Phase log n: Max # messages per leader Max # current leaders Phase 1: 4 Phase 2: 8 … Phase i: Phase log n: Total messages:

Can We Do Better? The O(n log n) algorithm is more complicated than the O(n2) algorithm but uses fewer messages in worst case. Works in both synchronous and asynchronous case. Can we reduce the number of messages even more? Not in the asynchronous model: Thr: Any async. LE algorithm requires Ω(n log n) messages.

An O(n) msgs. Synchronous Algorithm must be known (i.e., it is non-uniform) The node with smallest id is elected leader There are phases: each phase consists of n rounds If in phase k=0,1,,… there is a node with id k this is the new leader, and let it know to all the other nodes the algorithm terminates

Phase 0 (n rounds): no message sent 48 9 22 15 nodes 16 33 24 57

Phase 1 (n rounds): no message sent 48 9 22 15 nodes 16 33 24 57

… Phase 9 new leader 48 9 22 15 nodes 16 33 24 57

Phase 9 (n rounds): n messages sent new leader 48 9 22 15 nodes 16 33 24 57

Phase 9 (n rounds): n messages sent new leader 48 9 22 15 nodes 16 33 24 57 Algorithm Terminates

Phase 9 (n rounds): n messages sent new leader 48 9 22 15 nodes 16 33 24 57 Total number of messages:

Algorithm analysis Correctness: Easy to see  Message complexity: O(n), which can be shown to be optimal  Time complexity: O(n*m), where m is the smallest id in the ring  not bounded by any function of n  Other disadvantages: Requires synchronous start Requires knowing n (non-uniform)

Another Synchronous Algorithm Works in a slightly weaker model than the previous synchronous algorithm: processors might not all start at same round; a processor either wakes up spontaneously or when it first gets a message uniform (does not rely on knowing n) IDEA: messages travel at different “speed” (the leader’s one is the fastest)

Another Synchronous Algorithm A processor that wakes up spontaneously is active; sends its id in a fast message (one edge per round) A processor that wakes up when receiving a msg is relay; never in the competition A fast message carrying id m becomes slow if it reaches an active processor; starts traveling at one edge per 2m rounds A processor only forwards a msg whose id is smaller than any id it has previously sent If a proc. gets its own id back, elects self

Algorithm analysis Correctness: convince yourself that the active processor with smallest id is elected. Message complexity: Winner's msg is the fastest. While it traverses the ring, other msgs are slower, so they are overtaken and stopped before too many messages are sent.

Message Complexity Divide msgs into four types: fast msgs slow msgs sent while the leader's msg is fast slow msgs sent while the leader's msg is slow slow msgs sent while the leader is sleeping Next, count the number of each type of msg.

Number of Type 1 Messages (fast messages) Show that no processor forwards more than one fast msg: Suppose pi forwards pj 's fast msg and pk 's fast msg. When pk 's fast msg arrives at pj : either pj has already sent its fast msg, so pk 's msg becomes slow (contradiction) pj has not already sent its fast msg, so it never will (contradiction) Number of type 1 msgs is O(n). pk pj pi

Number of Type 2 Messages (slow msgs sent while leader's msg is fast) Leader's msg is fast for at most n rounds by then it would have returned to leader Slow msg i is forwarded n/2i times in n rounds Max. number of msgs is when ids are as small as possible (0 to n-1 and leader is 0) Number of type 2 msgs is at most ∑n/2i ≤ n n-1 i=1

Number of Type 3 Messages (slow msgs sent while leader's msg is slow) Maximum number of rounds during which leader's msg is slow is n*2L (L is leader's id). No msgs are sent once leader's msg has returned to leader Slow msg i is forwarded n*2L/2i times during n*2L rounds. Worst case is when ids are L to L+n-1 Number of type 3 msgs is at most ∑n*2L/2i ≤ 2n L+n-1 i=L

Number of Type 4 Messages (slow messages sent while leader is sleeping) Claim: Leader sleeps for at most n rounds. Proof: Indeed, it can be shown that the leader will awake after at most k≤n rounds, where k is the counter-clockwise distance in the ring between the leader and the closest active processor which woke-up at round 1 (prove by yourself!) Slow message i is forwarded n/2i times in n rounds Max. number of messages is when ids are as small as possible (0 to n-1 and leader is 0) Number of type 4 messages is at most ∑n/2i ≤ n n-1 i=1

Total Number of Messages We showed that: number of type 1 msgs is at most n number of type 2 msgs is at most n number of type 3 msgs is at most 2n number of type 4 msgs is at most n Thus total number of msgs is at most 5n=O(n).

Time Complexity Running time is O(n 2m), where m is smallest id. Even worse than previous algorithm, which was O(n m)  The advantage of having a linear number of messages is paid by both the algorithms with a number of rounds which depends on the minimum id 

Summary of LE algorithms on rings Anonymous rings: no any algorithm Non-anonymous asynchronous rings: O(n2) algorithm (unidirectional rings) O(n log n) messages (optimal, bidirectional rings) Non-anonymous synchronous rings: O(n) messages, O(nm) rounds (non-uniform, all processors awake at round 1) O(n) messages, O(n2m) rounds (uniform)

LE algorithms on general topologies Anonymous: no any algorithm (of course…) Non-anonymous asynchronous systems: O(|E|+n log n) messages Non-anonymous synchronous systems: O(|E|+n log n) messages, O(n log n) rounds Homework: think to complete graphs…

Exercise: Write the pseudo-code and execute the slow-fast algorithm on the following ring, assuming that p1, p5, p8 will awake at round 1, and p3 will awake at round 2. p1 p8 3 1 p2 5 p7 7 p3 2 11 p4 p6 8 6 p5